Field of the Invention and Description of Prior Art
This invention pertains to apparatus for measuring the quantity of organic matter in water. Such matter may include volatile organics including volatile halogenated organics such as trihalomethanes ("THM's).
Prior art apparatus for measuring organic matter in water includes:
Total Oxygen Demand ("TOD") Analyzer: This apparatus uses a furnace operating at about 900.degree. C. and containing for example
Pd or Pt wool or gauze. A stream of e.g. N.sub.2, He or A carrier gas containing a low level of O.sub.2 passes through the furnace. The O.sub.2 content of the carrier gas down-stream of the furnace is measured electrochemically, e.g. by a doped ZrO.sub.2 high temperature O.sub.2 concentration cell. From time to time a precise droplet of water is injected automatically or by manual micro-syringe directly into the furnace and the depletion of O.sub.2 in the carrier gas measured. The apparatus must be calibrated from time to time against a known standard sample. The apparatus suffers from instability of the O.sub.2 concentration cell sensor and from the fact that the latter responds not to change in O.sub.2 concentration but to the change in logarithm of such concentration. It responds not only to carbon compounds but also to compounds such as NH.sub.3. It responds "negatively" to N0.sub.3 - and dissolved 0.sub.2. The latter can be reduced to low levels by sparging with N.sub.2, He or A for example but at the risk of loss of volatile carbon compounds.
TOD is not sufficiently sensitive at concentrations of organics of interest in potable, pure and ultrapure water. Total Carbon Analyzer ("TCA"): This apparatus also uses typically a 900.degree. C. furnace with noble-metal wool or gauze. The gas passing through the furnace may be air though for analyses at low levels of carbon compounds in water it is preferred to use carbon-dioxide-free air or pure oxygen optionally diluted with an unreactive gas such as N.sub.2, He or A. The CO.sub.2 content of the gas downstream of the furnace is measured with a non-dispersive IR photometer. Again a precise droplet of water is injected from time to time into the furnace and the increase in CO.sub.2 in the gas stream determined. This apparatus must also be calibrated from time to time against known samples.
A variation of the apparatus reduces the CO.sub.2 to CH.sub.4 with hydrogen and measures the latter by flame ionization detection. Both variations suffer also from lack of sensitivity below 1 ppm organic carbon.
Organic Carbon Analyzer ("OCA" or "TOC"): The two variations of TCA discussed above measure not only CO.sub.2 from oxidation of organic carbon compounds but also from CO.sub.2, HCO.sub.3 - and/or CO.sub.3 = (inorganic carbon") which may be present in the water. An OCA device typically comprises a TCA apparatus having a pretreatment section to remove CO.sub.2, HCO.sub.3 =, for example by acidifying and stripping with air, N.sub.2 etc.. Such pretreatment may also remove volatile organic compounds ("VOC's") including THM's, thereby potentially further degrading the sensitivity of the apparatus.
Attempts have been made to overcome the problems occasioned in the above devices from the presence of the high temperature furnaces by oxidizing the organic matter in a sample in the liquid state, promoted by U.V. light with or without added persulfate salt. The CO.sub.2 produced has been detected in a variety of ways, for example:
a) the CO.sub.2 is sparged out of the oxidation zone typically with air (though O.sub.2, N.sub.2, A, He could in principle be used) and dissolved in a known volume of ultrapure water. The change in electrical conductivity of the ultrapure water is measured from which the concentration of CO.sub.2 can be calculated. However, as is well known, CO.sub.2 is present in water (at least in part) as H.sub.2 CO.sub.3, a very weak acid which is poorly ionized. Hence the change in conductivity of the ultrapure water receiving stream is small for low concentrations of organic carbon in the sample. Such defect may be ameliorated in part by using fairly large samples compared to the volume of receiving ultrapure water. Any strongly ionized electrolytes carried over as mist in the CO.sub.2 sparging gas of course give a spurious result. In one version of the above described apparatus the sparging gas is recycled to the oxidation zone in which case an equilibrium is eventually reached between dissolved free (i.e. uncombined) CO.sub.2 in the oxidation zone (at the pH pertaining therein) and in the ultrapure water. There is increased risk in this case of carry-over of strong electrolytes. To distinguish between inorganic and organic carbon compounds the sample in the oxidation zone is acidified before
oxidation commences and the resulting (inorganic) CO.sub.2 removed to the receiving ultrapure water by sparging. VOC's may be removed in the process. (This practice tends to mask the effects of carry-over of strong electrolytes). The apparatus is not stable and both background correction and calibration must be carried out on at least a daily basis if samples containing 1 ppm or less of organic carbon are to be analyzed
FIG. 1 is a schematic representation of one embodiment of apparatus according to the above described process. The major components of the apparatus are:
the U.V. irradiation chamber 201 with a 184.9 nm U.V. light source 202 surrounded by a quartz or other 184.9 nm substantially transparent sleeve.
the measuring chamber 203 which houses a conductivity cell 204. (The latter according to the present invention preferably comprises a weakly basic anion exchange structure as more fully described below).
an eductor 205 which pulls a gas e.g. air from the irradiation chamber 201 through the measuring chamber 203.
a pump 206.
a mixed bed ion exchanger 207 to purify the water in measuring chamber 203 after each analysis.
a conductivity processing and display and/or recording module 208.
Analysis begins with closing of valves on each end of the mixed bed ion exchanger 207, de-energizing UV light source 202 and introducing a suitable volume of sample plus a small amount of e.g. phosphoric acid and optionally potassium peroxydisulfate into U.V. irradiation chamber 201. Throughout the analysis water is recirculated through the pump 206, eductor 205 and U.V. irradiation chamber 201 and a stream of suitable gas from U.V. irradiation chamber 201 through the water in the measuring chamber 203 back through the eductor 205 into U.V. irradiation chamber 201. A first conductance of conductivity cell 204 is then measured and the U.V. light source is then energized. When the conductance of conductivity cell 204 essentially reaches steady state a second conductance of such cell is measured. The difference between such first and second conductances is a measure of the organic carbon content of the above mentioned sample. (According to the improvement described herein below conductivity cell 204 is preferably a weakly basic anion exchange resin). The liquid water in measuring chamber 203 is then recirculated through mixed bed ion exchanger 207 thoroughly to deionize the water.
b) the CO.sub.2 is sparged out of the oxidation zone, trapped and concentrated on a solid sorbent. The sorbent is subsequently heated rapidly and the CO.sub.2 released is measured by a non-dispersive lR detector. In principle the released CO.sub.2 could be catalytically hydrogenated to CH.sub.4 and the latter detected by flame ionization. Inorganic and organic carbon are distinguished by acidifying and sparging the sample before the oxidation step. VOC's may be removed in the latter process. In some versions of the above described device the step of concentrating the CO.sub.2 on a solid sorbent is omitted.
c) the change in electrical conductivity of the water in the oxidation zone is measured and interpreted in terms of CO.sub.2 or organic carbon. The sparging step is thereby eliminated. In a variation of this method oxidation of the organics in the sample is allowed to proceed only to organic acids and not to CO.sub.2. In either case the change in conductivity must be detected against the background of electrolytes initially present in the sample. Accurate results are difficult to obtain if such background conductivity is greater than about 2 micro-Siemens/cm (".mu.S/cm") i.e. about 1 ppm NaCl. Since the measurement of conductivity change is carried out in the same volume of water in which the UV promoted oxidation takes place, correction is not required for inorganic carbon. It is clear however that the presence of HCO.sub.3 - (or anions of other weak inorganic acids) must buffer the dissociation of any CO.sub.2 (and/or organic acid) produced by the oxidation. Further it is clear that some organics are easily and rapidly oxidized to CO.sub.2 by UV promoted oxidation and that it may be difficult to limit the oxidation products to organic acids. It is equally clear that other organics are oxidized to CO.sub.2 only slowly and with difficulty by UV promoted oxidation. Such devices are therefore less useful
in monitoring the absolute concentration of organics in an unidentified water sample than in monitoring the hour-by-hour or day-by-day performance of a purification system on a given water source. Hydrohalic acids produced by oxidation of halogenated organics also interfere with the interpretation of the conductivity charge.
FIG. 2 is a schematic representation of one embodiment of apparatus according to the above described process. 510 represents a sample cell arranged to be connected at port 512 to a source of influent water which is to be analyzed for organic carbon. Effluent water exits at port 514. Cell 510 is divided into two main parts 516 and 518. A recess in part 516 is covered by a quartz window (or other U.V. transparent window). A liquid tight chamber 524 is formed in part 516 by gasket 522 and window 520. The conductivity of the liquid in chamber 524 is measured by means of central electrode 526 and peripheral electrode 528 (According to the improvement described herein below the concentration of carbonaceous acids is measured by means of the conductivity of weakly basic anion exchange resin, preferably shielded from the U.V. irradiation). A temperature sensor 527 may be attached to the rear of chamber 524. Electrodes 526 and 528 may be connected to an analog-digital converter 530 (or to a simple conductivity meter) and to a data processing and/or recording device 532. A U.V. lamp 534 is inserted into port 518. 536 represents a mirror. Chamber 538 is generally filled with a gas which does absorb the principal lines of lamp 534.
It is obvious that the sample of water to be analyzed by the devices mentioned immediately above (i.e. in which the conductivity after oxidation is measured in the same medium) may not be acidified and sparged to remove inorganic carbon since the added acid contributes an unacceptable background electrical conductivity and will as well suppress ionization of any CO.sub.2 (or organic acids) formed in the oxidation step. (It will be understood however that owing to the nature of the detection process inorganic carbon will not interfere with the detection of organic carbon except for the buffering effect of the former mentioned above). If the sample contains halogenated organics oxidation of which is promoted by UV, then the conductivity contributed by any hydrohalo acids formed will be interpreted as a relatively very much larger concentration of CO.sub.2 (or organic acid).
It will be seen from the above discussion that the problem of measuring low levels of organic compounds in water in the presence of inorganic carbon (CO.sub.3 =, HCO.sub.3 - and H.sub.2 CO.sub.3), VOC's (including volatile halogenated organics) and/or significant amounts of electrolytes has not been satisfactorily solved by methods and apparatus known in the art. In U.S. Pat. No. 4,940,667 (assigned to the same assignee as the present application and incorporated herein by reference) an improvement in the art was disclosed in which at least part of a sample of water to be analyzed is vaporized by "quiet boiling"through a high temperature oxidizing/reforming furnace which oxidizes/reforms VOC's into C0.sub.2. The vaporized water and CO.sub.2 formed are condensed and the electrical impedance of the thus condensed liquid is measured. The method is simple and inexpensive and, if care is taken to avoid carry-over of mist, suppresses the contribution to the electrical conductivity of the condensed sample from strongly ionized inorganic electrolytes present in the sample. (Correction may be made for any residual contribution of such electrolytes and volatile inorganic carbon by running a duplicate sample in the apparatus with the temperature of the furnace below oxidation/reforming temperatures). The method and apparatus of said application do not detect non-volatile organics however and do not distinguish between C0.sub.2 formed from VOC's and hydrohalic acids formed from volatile halogenated organics. Precision of the method and apparatus depends upon measuring or controlling the fraction of the sample which is both vaporized by quiet boiling and condensed to liquid water and by substantial condensation and reflux to the quiet boiler of vapor before said vapor enters the high temperature oxidizing/reforming furnace. It goes without saying that the temperature of the impedance measuring sensor must be controlled and/or measured.
The apparatus described above as shown in FIG. 3 is taken from said U.S. Pat. No. 4,940,667. The apparatus is fabricated in whole or in part from commercial quartz, vitreous silica, pyroceram, alumina, mullite or porcelain. Chamber 41 is a receiver typically having a diameter of about 11 to 12 millimeters and a height of from about 37 to about 50 millimeters. Receiver 41 may be wrapped in Nichrome or similar heating wire held in place with a high temperature ceramic adhesive well known in the art. Water to be analyzed is introduced into receiver 41 by conduit means 42a and/or 42b. Vapor from receiver 41 passes into chamber 413 which serves the dual purpose of mist eliminator and partial condenser. In the embodiment shown in FIG. 4, chamber 413 protrudes into receiver 41 and has openings 415 to allow vapor to enter chamber 413 while minimizing the probability of liquid being entrained in case of bumping during evaporation in receiver 41. Any condensate formed in chamber 413 flows back into receiver 41 through openings 415. Such partial condensation of vapor (caused by cooling at walls of chamber 413) enriches the concentration of volatile organics (particularly volatile, less hydrophilic organics) in the water vapor passing from chamber 413 into heated zone 45. Chamber 413 may be at least partly filled with suitable foraminous packing such as glass beads 414 to encourage good contact between refluxing partial condensate and vapor rising from receiver 41 toward heated zone 45. The flow of condensed water vapor counter-current to the ascending water vapor results in enrichment of organic components more volatile than water and also in scrubbing out of mist which may form in the event of less than gentle evaporation in receiver 41. Zone 45 may be heated by Nichrome or other suitable heating wire or ribbon held in place by a ceramic adhesive well known in the art. Zone 45 is generally insulated with suitable insulating material. Zone 45 is typically heated to a temperature in the range of 450.degree. to 1000.degree. C. at which temperature organic components in the water vapor are oxidized (by residual oxygen in the water vapor) and/or reformed (by the water vapor per se) into carbon dioxide and other simple compounds. The rate of oxidation and/or reformation may be increased by including extended surface area (such as porous ceramics or activated porous ceramics) in Zone 45. 47 represents condensing means, appropriately cooled, and may also contain extended surface, e.g. glass beads. A condensate receiver 410 communicates with impedance measuring means 48 which according to U.S. Pat. No. 4,940,667 may be a pair of electrodes (but according to the improvement described herein below is preferably a weakly basic anion exchange resin).
By way of improvement over the above mentioned method and apparatus, the organics in a sample of water are first oxidized and at least part (preferably a controlled and/or measured fraction) of the oxidized sample is vaporized by quiet boiling and condensed, preferably with substantial reflux to the quiet boiler. Relevant apparatus is illustrated schematically in FIG. 4 in which 301 represents an entrance conduit for a liquid sample which has been oxidized (e.g. by ultraviolet irradiation with or without added oxidizing agents such as persulfate, by Fenton's reagent (hydrogen peroxide plus an iron salt), by persulfate plus a silver salt at an elevated temperature, by ozone with or without added hydrogen peroxide, by chromic acid etc.) and which has a pH preferably less than about 4.5, for example by having been adjusted with phosphoric acid. 304 represents a reboiler, heated with any suitable element 305 well known in the art. 302 represents a volatile carbonaceous acid enriching section, packed with any suitable material, e.g. glass beads, and 303 represents a volatile carbonaceous acid stripping section also packed with suitable material. (Such stripping section may be omitted under some circumstances). Distillate issuing from the top of 302 is condensed by condenser 307 and collected in receiver 308. The electrical impedance of condensed water in 308 is measured for example by high frequency coil 310 or by a pair of suitable electrodes (but according to the improvement described herein below is preferably a weakly basic anion exchange resin). Underflow from receiver 308 through conduit 311 provides reflux to the carbonaceous acid fractionator represented by 302 and 303. In one embodiment reboiler 304 is also the oxidation zone, in which case stripping section 303 may be eliminated and the water sample, added oxidant and added pH control agents (if any) introduced through 301 directly into reboiler 304. The electrical impedance of the condensed liquid is measured. The improvement effectively suppresses the contribution to the electrical conductivity of the condensed sample by inorganic electrolytes present in the sample. Hence peroxydisulfates and other oxidizing agents (e.g. iron salts plus hydrogen peroxide), buffering agents, acids can be added to the sample before or during oxidation and/or before or during quiet boiling to enhance the oxidation or for other effects useful in the analysis. The oxidizing agents can include ozone generated on site. The oxidation can be assisted by UV irradiation.
The above mentioned improvement also suppresses the interference caused by chlorine containing organics since any hydrogen chloride formed by oxidation of the latter is much less volatile than water vapor at low concentrations of hydrogen chloride.
All the above mentioned methods and apparatus which determine CO.sub.2 formed from organic carbon by change in electrical conductivity or impedance caused by such CO.sub.2 suffer from the poor dissociation of CO.sub.2 (resp. H.sub.2 CO.sub.3) in water and hence from loss of precision owing to possible contamination by strongly dissociated electrolytes. It is therefore an objective of the present invention to provide a simple, inexpensive method and apparatus for detecting CO.sub.2 which is not restricted by such poor dissociation. It is also an objective to provide an apparatus and method suitable for the detection of low concentrations of organics in water particularly in potable, pure and ultrapure water, which method and apparatus avoid many of the problems of prior art apparatus and methods. These and other objectives will become apparent from the disclosure and claims below.